Docs: Link to Jinja 2.8.1 documentation mirror at jinja.octoprint.org

The official docs are only 2.9+, that doesn't help us as long as we are
stuck with 2.8.

Fixes #1900

docs/features/gcode_scripts.rst

@@ -13,11 +13,19 @@ Unless :ref:`configured otherwise <sec-configuration-config_yaml-folder>`, OctoP
 the ``scripts/gcode`` folder in OctoPrint configuration directory (per default ``~/.octoprint`` on Linux, ``%APPDATA%\OctoPrint``
 on Windows and ``~/Library/Application Support/OctoPrint`` on MacOS).
 
-These GCODE scripts are backed by the templating engine `Jinja2 <http://jinja.pocoo.org/>`_, allowing more than just
+These GCODE scripts are backed by the templating engine Jinja2, allowing more than just
 simple "send-as-is" scripts but making use of a full blown templating language in order to create your scripts. To
 this end, OctoPrint injects some variables into the :ref:`template rendering context <sec-features-gcode_scripts-context>`
 as described below.
 
+You can find the docs on the Jinja templating engine as used in OctoPrint at `jinja.octoprint.org <http://jinja.octoprint.org/templates.html>`_.
+
+.. note::
+
+   Due to backwards compatibility issues with Jinja versions 2.9+, OctoPrint currently only supports Jinja 2.8. For this
+   reason use the template documentation at `jinja.octoprint.org <http://jinja.octoprint.org/templates.html>`_ instead of the
+   documentation of current stable Jinja versions.
+
 .. _sec-features-gcode_scripts-predefined:
 
 Predefined Scripts
@@ -150,6 +158,7 @@ to 0 if a heated bed is configured.
 
 .. seealso::
 
-   `Jinja Template Designer Documentation <http://jinja.pocoo.org/docs/dev/templates/>`_
+   `Jinja Template Designer Documentation <http://jinja.octoprint.org/templates.html>`_
       Jinja's Template Designer Documentation describes the syntax and semantics of the template language used
-      also by OctoPrint's GCODE scripts.
+      also by OctoPrint's GCODE scripts. Linked here are the docs for Jinja 2.8.1, which OctoPrint still
+      relies on for backwards compatibility reasons.

docs/plugins/gettingstarted.rst

@@ -490,7 +490,7 @@ Also adjust your plugin's ``templates/helloworld_navbar.jinja2`` like this:
 
 OctoPrint injects the template variables that your plugin defines prefixed with ``plugin_<plugin identifier>_`` into
 the template renderer, so your ``url`` got turned into ``plugin_helloworld_url`` which you can now use as a simple
-`Jinja2 Variable <http://jinja.pocoo.org/docs/dev/templates/#variables>`_ in your plugin's template.
+`Jinja2 Variable <http://jinja.octoprint.org/templates.html#variables>`_ in your plugin's template.
 
 Restart OctoPrint and shift-reload the page in your browser (to make sure you really get a fresh copy). The link should
 still work and point to the URL we defined as default.
@@ -1078,8 +1078,14 @@ For some insight on how to create plugins that react to various events within Oc
 add support for a slicer, OctoPrint's own bundled `CuraEngine plugin <https://github.com/foosel/OctoPrint/wiki/Plugin:-Cura>`_
 might give some hints. For extending OctoPrint's interface, the `NavbarTemp plugin <https://github.com/imrahil/OctoPrint-NavbarTemp>`_
 might show what's possible with a few lines of code already. Finally, just take a look at the
-`list of available plugins <https://github.com/foosel/OctoPrint/wiki#plugins>`_ on the OctoPrint wiki if you are
-looking for examples.
+`official Plugin Repository <http://plugins.octoprint.org>`_ if you are looking for examples.
+
+.. seealso::
+
+   `Jinja Template Designer Documentation <http://jinja.octoprint.org/templates.html>`_
+      Jinja's Template Designer Documentation describes the syntax and semantics of the template language used
+      by OctoPrint's frontend. Linked here are the docs for Jinja 2.8.1, which OctoPrint still
+      relies on for backwards compatibility reasons [#f3]_.
 
 .. rubric:: Footnotes
 
@@ -1092,3 +1098,7 @@ looking for examples.
 .. [#f2] Refer to the `LESS documentation <http://lesscss.org/#using-less>`_ on how to do that. If you are developing
          your plugin under Windows you might also want to give `WinLESS <http://winless.org/>`_ a look which will run
          in the background and keep your CSS files up to date with your various project's LESS files automatically.
+.. [#f3] Please always consult the Jinja documentation at `jinja.octoprint.org <http://jinja.octoprint.org>`_ instead of
+         the current stable documentation available at Jinja's project page. The reason for that is that for backwards
+         compatibility reasons OctoPrint currently sadly has to rely on an older version of Jinja. The documentation
+         available at `jinja.octoprint.org <http://jinja.octoprint.org>`_ matches that older version.